Ferroelectric liquid-crystalline mixtures

ABSTRACT

The novel ferroelectric liquid-crystalline mixtures are based on at least two mixture components of the general formula (I) ##STR1## but additionally contain at least one optically active dope from one or more of the following groups 
     a) optically active esters of tetrahydrofuran-2-carboxylic acid 
     b) optically active aryl 2,3-epoxyalkyl ethers 
     c) optically active 1,3-dioxolan-4-yl ethers. 
     The ferroelectric LC mixtures have a high spontaneous polarization and facilitate substantial compensation of the pitch of the helix in a nematic phase.

Ferroelectric liquid crystals have recently attracted interest as adisplay medium in electrooptical assemblies (for example Lagerwall etal., "Ferroelectric Liquid Crystals for Displays", SID Symposium,October Meeting 1985, San Diego, Calif. USA).

For practical use of ferroelectric liquid crystals in electroopticaldisplays, chiral, tilted, smectic phases, such as S_(c) phases, arerequired [R. B. Meyer, L. Liebert, L. Strzelecki and P. Keller, J.Physique 36, L-69 (1975)], which are stable over a broad temperaturerange. This aim can be achieved by means of compounds which themselvesform such phases, for example S_(c) phases, or, however, by dopingcompounds which form non-chiral, tilted, smectic phases, with opticallyactive compounds [M. Brunet, C. Williams, Ann. Phys. 3, 237 (1978)].

Furthermore, a unitary planar orientation of the liquid crystals isnecessary in order to achieve a high contrast ratio when ferroelectricliquid-crystal mixtures are used in electrooptical assemblies. It hasbeen shown that a unitary planar orientation in the S_(c) phase can beachieved if the phase sequence of the liquid-crystal mixture withdecreasing temperature is as follows:

    Isotropic→nematic→smectic A→smectic C.

(for example K. Flatischler et al., Mol. Cryst. Liq. Cryst. 131, 21(1985); T. Matsumoto et al., p. 468-470, Proc. of the 6th Int. DisplayResearch Conf., Japan. Display, Sep. 30-Oct. 2, 1986, Tokyo, Japan; M.Murakami et al., ibid., p. 344-347).

For ferroelectric (chiral, smectic) liquid-crystal mixtures, thecondition that the pitch of the helix must be large, i.e. greater than 5μm, in the S_(c) * phase and must be very large, i.e. greater than 10 μmor infinite, in the N* phase must additionally be fulfilled.

The optical switching time τ[μs] of ferroelectric liquid-crystalsystems, which should be as short as possible, depends on the rotationviscosity of the system γ[mPas], the spontaneous polarization P_(s)[nC/cm² ] and the electric field strength E[V/m], in accordance with theequation ##EQU1## Since the field strength E is determined by theelectrode separation in the electrooptical assembly and by the appliedvoltage, the ferroelectric display medium must have low viscosity andhigh spontaneous polarization in order that a short switching time isachieved.

Finally, besides thermal, chemical and photochemical stability, a smalloptical anisotropy Δn, preferably <0.13, and a small positive or,preferably, negative dielectric anisotropy Δε are required. (S. T.Lagerwall et al., "Ferroelectric Liquid Crystals for Displays" SIDSymposium, October Meeting 1985, San Diego, Calif. USA).

It is only possible to satisfy all these demands using mixturescomprising several components. The basis (or matrix) used here ispreferably compounds which if possible themselves already have thedesired phase sequence I→N→S_(A) →S_(c). Further components of themixture are often added to reduce the melting point and to broaden theS_(c) and usually also the N phase, to induce optical activity, forpitch compensation and to match the optical and dielectric anisotropy;however, the rotation viscosity, for example, should if possible not beincreased at the same time.

Some of these components and also certain mixtures are already knownfrom the prior art. However, since the development, in particular offerroelectric liquid-crystal mixtures, can in no way be regarded ascomplete, the manufacturers of display elements are interested in a verywide variety of mixtures. A further reason for this, amongst others, isthat conclusions on the quality of the liquid-crystalline mixtures toocan only be made by considering the interactions of theliquid-crystalline mixtures with the individual assemblies of thedisplay elements or of the cells (for example the orientation layer).

It is known that certain derivatives of phenylpyrimidine, in particular5-alkyl-2(4-alkoxyphenyl)pyrimidines, are capable of forming S_(c),S_(A) and N phases (D. Demus and H. Zaschke, "Flussige Kristalle inTabellen", [Liquid Crystals in Tables], VEB Deutscher Verlag furGrundstoff-industrie, Leipzig 1974, pp. 260-261) and, in addition, canbe converted into ferroelectric liquid-crystal mixtures by addition ofoptically active dopes [L. M. Blinov et al., Sow. Phys. Usp. 27 (7), 492(1984); L. A. Beresnew et al., Ferroelectrics, 59 [321]/1 (1984),presented at the 5th Conference of Soc. Countries on Liquid Crystals,Odessa, USSR, October 1983; DE-A-3,515,347, EP-A 0,206,228 and EP-A0,225,195].

It is furthermore known that lower melting points and a broadening ofthe liquid-crystalline phases desired can be achieved by mixing severalliquid-crystalline compounds [D. Demus et al., Mol. Cryst. Liq. Cryst.25, 215 (1974), J. W. Goodby, Ferroelectrics 49, 275 (1983)], and thatthe melting-point depression is the more pronounced the more the mixturecomponents also differ structurally. (J. S. Dave et al., J. Chem. Soc.1955, 4305). It was thus to be presumed that particularly low mixturemelting points, and thus low phase-transition points of the S_(c) phase,are obtained when compounds are mixed which, on the one hand, aresignificantly different structurally, but, on the other hand, aresufficiently similar to be readily miscible.

DE-C 2,257,588 discloses a 5-butoxy-2(4-pentoxyphenyl)pyrimidine, butthis only forms a nematic phase.

In DE 3,831,226.3, it has been found that, as the base component,compounds of the type 5-alkoxy-2(4-alkoxyphenyl)pyrimidines of thegeneral formula (I) ##STR2## in which C_(n) H_(2n+1) and C_(x) H_(2x+1)are straight-chain alkyl radicals in which n is an integer from 6 to 14and x is an integer from 2 to 14, are particularly highly suitable forliquid-crystalline mixtures, in particular ferroelectricliquid-crystalline mixtures; of them, at least two different compoundsa' and a" are present in the mixture. For the most part, they exhibitthe desired phase sequence I→N→S_(A) →S_(c) with decreasing temperature.Compared with the known 5-alkyl-2-(4-alkoxyphenyl)pyrimidines, they haveconsiderably broader S_(c) phases and higher transition temperatures ofthe S_(c) →S_(A) transition. In addition, the S_(c) phase is even formedin the case of a lower number of carbon atoms in the alkyl(oxy) chains,for example even when n=8 and x≯2 or n=6 and x≯6. Compared with theknown5-alkyl-2-(4-alkoxyphenyl)pyrimidines, this gives a greater choiceof homologous, readily miscible compounds having S_(c) phases forpreparing the mixture. Due to their higher phase-transition temperaturesS_(A) to S_(c) (for example 70° to 90° C.) and their broader S_(c)phases, they also make it possible to prepare mixtures having very broadS_(c) phases (for example up to a phase width of about 68°) and hightransition temperatures S_(A) /S_(c).

The mixtures of DE 3,831,226.3 of compounds of the formula (I) arealready very highly suitable as a matrix for ferroelectricliquid-crystalline mixtures. However, they also have a large opticalanisotropy (Δn) and a large positive dielectric anisotropy (Δε). Inaddition, the nematic phase, depending on the chain length n or x of thesubstituents, is still narrow.

By adding at least one compound (cyclohexanecarboxylate) of the generalformula (II), i.e. a compound of type b, ##STR3## the parameters [Δn]and [Δε] can simultaneously be optimized and the width of the nematicphase can be matched to practical requirements.

In the general formula (II): R¹ is an alkyl chain having 10 to 16 carbonatoms or an alkoxy chain having 8 to 14 carbon atoms, and R² is an alkylchain having 2 to 9 carbon atoms. These compounds are described inDE-A-3,731,639. They develop their favorable action even when added inamounts from about 2 mol %, relative to the mixture of the components oftype a and b. When added in amounts from about 10 and up to about 40 mol%, the nematic phase is broadened by about 8° to 18° C. and the smecticphase by about 6° to 15° C. In addition, it is very advantageous thatthe addition only insignificantly increases the rotation viscosity ofthe base mixture according to the invention, which is already very lowper se.

The melting point and the lower phase-transition temperature of theS_(c) phase of the mixtures can be reduced, if they are still too high,by adding at least one compound (alkenyloxyphenylpyrimidine derivative)of the general formula (III), i.e. a compound of type c, to mixtures ofcompounds of type a alone or additionally containing compounds of typeb: ##STR4##

In the general formula (III), R³ is a branched or straight-chain alkylradical having 7 to 16 carbon atoms or a straight-chain or branchedalkoxy radical having 6 to 14 carbon atoms, and y is an integer from 4to 14. These compounds are described in DE-A-3,731,638. Added amounts offrom about 10 to 35 mol %, relative to the overall mixture, reduce thelower temperature limit of the S_(c) phase by up to 5° C. The otherfavorable physical properties of the mixtures are generally not impairedby this addition.

To reduce the melting point and the lower temperature limit of the S_(c)phase, it is also possible to add, together with or in place ofcompounds of type c, compounds of the formula (IV), i.e. of type d,##STR5## where m and p, independently of one another, are integers from6 to 14. When amounts of from about 10 to 25 mol % of the compound oftype d or mixtures with compounds of type c, relative to the overallmixture, are added to mixtures of compounds of type a or a and b, thelower temperature limit of the S_(c) phase is reduced by up to 10° C.

To further reduce the melting point and to improve the optical anddielectric anisotropy, and thus the contrast, in a display, thefollowing carboxylates have furthermore been proposed as mixturecomponents for a liquid-crystalline base mixture:

Carboxylates C of the general formula (V), (VI), (VII), (VIII) or (IX)##STR6## where k may be an integer from 6 to 14, and

l may be an integer from 2 to 14, ##STR7## where m may be an integerfrom 5 to 14, and

l may be an integer from 2 to 14, ##STR8## where p may be an integerfrom 7 to 14, and

r may be an integer from 4 to 14, ##STR9## where s may be an integerfrom 6 to 14, and

t may be an integer from 6 to 14, ##STR10## where R² is straight-chainor branched (C₁ -C₁₂)alkyl or alkenyl, it being possible for one or twonon-adjacent CH₂ groups to be replaced by O and/or S atoms,

y is F, Cl, Br, CN or CF₃, and

R¹ is branched (C₃ -C₉)alkyl, benzyl or phenyl.

The object of the present invention is to develop novel ferroelectric LCmixtures which have high spontaneous polarization [P_(s) ], lowviscosity and good orientability in a display element by means ofparticularly suitable dopes in liquid-crystalline base mixtures.

The invention proceeds from base mixtures presented in DE 3,831,226 andin DE-A-3,909,356. However, the novel mixtures according to theinvention additionally contain a chiral, optically active component D asdope, which results in a large pitch of the helix in the N* phase of themixture and induces high spontaneous polarization (P_(s)) even in lowconcentration.

The addition of small amounts of dopes which result in high spontaneouspolarization is therefore advantageous since the viscosity of theoverall FLC mixture is not significantly increased.

The use of two dopes D which both have the same sign of spontaneouspolarization, but have a different helix rotation sense, is particularlyadvantageous. On compensation of the pitch, this causes a synergisticeffect in increasing the spontaneous polarization.

The invention thus also relates to dope combinations which have thissynergistic effect in the base mixtures mentioned.

The novel invention thus relates to a ferroelectric liquid-crystallinemixture containing, as component A, at least two5-alkoxy-2-(alkoxyphenyl)pyrimidines of the general formula (I)##STR11## in which the alkyl radicals are straight-chain alkyl radicalsin which n is an integer from 6 to 14 and x is an integer from 2 to 14,and, if appropriate, one or more cyclohexanecarboxylates of the generalformula (II) ##STR12## in which R¹ is an alkyl radical having 10 to 16carbon atoms or an alkoxy radical having 8 to 14 carbon atoms, and R² isan alkyl radical having 2 to 9 carbon atoms, and, if appropriate, one ormore alkenyloxphenylpyrimidine derivatives of the general formula (III)##STR13## in which R³ is an alkyl chain having 7 to 16 carbon atoms oran alkoxy chain having 6 to 14 carbon atoms, and y is an integer from 4to 14, and, if appropriate, one or more alkylpyrrimidinealkoxyphenylderivatives of the general formula (IV) ##STR14## in which m and p,independently of one another, are integers from 6 to 14, and ifappropriate as component B, an optically active compound from the group

a) optically active esters made from α-chlorocarboxylic acids andmesogenic phenols,

b) optically active esters of N-acylproline,

c) optically active esters of 1,3-dioxolane-4-carboxylic acids, and

d) optically active esters of oxirane-2-carboxylic acids,

and if appropriate containing, as component C, one or more carboxylatesof the general formula: (V), (VI), (VII), (VIII) or (IX) ##STR15## wherek may be an integer from 6 to 14, and

l may be an integer from 2 to 14, ##STR16## where m may be an integerfrom 5 to 14, and

l may be an integer from 2 to 14, ##STR17## where p may be an integerfrom 7 to 14, and

r may be an integer from 4 to 14, ##STR18## where s may be an integerfrom 6 to 14, and

t may be an integer from 6 to 14, ##STR19## where R² is straight-chainor branched (C₁ -C₁₂)alkyl or alkenyl, it being possible for one or twonon-adjacent CH₂ groups to be replaced by O and/or S atoms,

y is F, Cl, Br, CN or CF₃ and

R¹ is branched (C₃ -C₉)alkyl, benzyl or phenyl, wherein theferroelectric liquid-crystal mixture furthermore contains an opticallyactive component D which comprises one or more compounds from thefollowing groups (X) and/or (XI) and/or (XII),

Group (X): optically active esters of tetrahydro-furan-2-carboxylic acidwith mesogenic phenols

Group (XI): optically active aryl 2,3-epoxyalkyl ethers

Group (XII) : optically active 1,3-dioxolan-4-yl ethers.

A preferred ferroelectric LC mixture is one which contains 0.5 to 30mol-% of the optically active component D.

Another preferred mixture is one which contains two dopes from Groups Dand B and in which the pitch of the helix induced in the nematic phaseis approximately compensated at a given temperature.

A particularly preferred FLC mixture is one which contains, as componentD, a chiral tetrahydrofuran-2 carboxylate (X) of the following generalstructure (Xa): ##STR20## where R² is a straight-chain or branched alkylor alkenyl radical having 6 to 12 carbon atoms, which may contain anasymmetric carbon atom;

-M³ is --O, --S, --O--CO or --CO,

k is zero or 1,

-A⁴ is ##STR21##

A further particularly preferred FLC mixture is one which contains, ascomponent D, a chiral epoxyalkyl ether of the general structure (XIa)##STR22## (aryl 2,3-epoxyalkyl ether) R¹ is straight-chain or branched(C₁ -C₁₂)alkyl, it being possible for one or two non-adjacent CH₂ groupsto be replaced by O and/or S atoms,

A is diazine-2,5-diyl or diazine-3,6-diyl, and

R², R³ and R⁴, independently of one another, are H, straight-chain (C₁-C₁₀)alkyl or branched (C₃ -C₁₀)alkyl, where R², R³ and R⁴ are notsimultaneously H,

and an FLC mixture which contains, as component D, a chiraldioxolan-4-yl ether of the structure (XIIa) ##STR23## in which: R⁶ andR⁷ are methyl or, together with the C(2) atom of the dioxolane ring, area cyclohexane ring,

R⁵ is a straight-chain or branched alkyl or alkylene radical having 6 to12 carbon atoms, which may contain an asymmetric carbon atom,

-M³ is --O, --S or --O--CO,

k is zero or 1, and

-A⁴ is ##STR24##

Very particularly preferred ferroelectric liquid-crystal mixtures arethose which contain one or more compounds of type (Xb) and/or (XIb) and,a (XIIb) as component D: ##STR25## where A is pyrimidine-2,5-diyl, and

R¹ is C₁ -C₁₀ -alkyl, it being possible for one or two non-adjacent CH₂groups to be replaced by O.

The ferroelectric liquid-crystal mixtures according to the invention mayadvantageously be employed in electrooptical assemblies and displayelements, in particular because they result in high contrast and a shortswitching time.

d₁) The optically active esters of tetrahydrofurancarboxylic acid (X)with an (R)-configuration induce a positive spontaneous polarization anda positive helix rotation sense in the mixtures according to theinvention.

Optically active 1,3-dioxolane-4-carboxylates having an(R)-configuration are suitable for the compensation, since they induce apositive P_(s) and such a large negative pitch that only 1 part byweight of the dioxolane derivative need be employed per 3 parts byweight of tetrahydrofurancarboxylic acid for compensation.

d₂) Likewise, optically active epoxyalkyl ethers (XI) may be employedfor pitch compensation in combination with thetetrahydrofurancarboxylates (X), but in this case the two componentsmust be employed in an approximately equal weight ratio.

e₁) Optically active aryl 2,3-epoxyalkyl ethers (XI) having a(2S,3S)-configuration induce in the mixtures according to the inventiona positive spontaneous polarization and negative pitch, which can becompensated by adding optically active oxirane-2-carboxylates (as inDE-A-3,718,174) having a (2R,3R)-configuration. This class of substancesis particularly suitable for pitch compensation, since only 1 part byweight of oxiranecarboxylate is generally required per 3 parts by weightof aryl 2,3-epoxyalkyl ether (XI).

f₁) Optically active ethers of 1,3-dioxolane derivatives having an Rconfiguration induce a negative pitch in the mixtures according to theinvention. They are therefore suitable for compensation of the mixturescontaining optically active oxirane-2-carboxylates having a positiveP_(s) and a (2R,3R)-configuration. Due to the small pitch of thedioxolane ethers, relatively small amounts of this substance arerequired for pitch compensation. A weight ratio of these dopes of about1:1 has proven successful for pitch compensation.

g) The optically active aryl 2,3-epoxyalkyl ethers having a(2S,3S)-configuration may also preferably be replaced for pitchcompensation in the pitch-compensated dope combinations from Groups e₁and d₂ by the compounds having a (2S,3R)-configuration. It isadvantageous here that the compound having a (2S,3R)-configurationinduces in the mixtures according to the invention a higher spontaneouspolarization and such a small helix that only about half the amount ofaryl 2,3-epoxyalkyl ethers having a (2S,3R)-configuration is necessaryfor pitch compensation compared with the combinations mentioned.

Thus, it has been found, surprisingly, that the level of about 10 mol-%of an optically active tetrahydrofuran-2-carboxylate (X) results in themixtures of DE 3,831,226 and in DE-A-3,909,356 in a broadening of themelting point of up to 5° C. and induces a spontaneous polarization ofup to +30 nC.cm-2.

The values for

the spontaneous polarization P_(s) [nC/cm² ],

the contrast K,

the optical switching time τ[μs],

the pitch of the helix in the N* phase [μm],

the dielectric anisotropy Δε

and the optical anisotropy Δn are determined for the ready-to-useferroelectric liquid-crystal mixtures (all measurements at a temperatureof 25° C.).

The P_(s) values are measured by the method of H. Diamant et al. (Rev.Sci. Instr., 28, 30, 1957), where measurement cells with an electrodeseparation of 2 μm and ground polyimide as the orientation layer areused.

To determine τ and K, the measurement cell is clamped between crossedanalyzer and polarizer on the rotary stage of a polarizing microscope.

To determine the contrast [K], the measurement cell is positioned, byrotation, so that a photodiode indicates minimum light transmission(dark state).

The microscope illumination is adjusted so that the photodiode indicatesthe same light intensity for all cells.

After a switching operation, the light intensity changes (bright state),and the contrast is calculated from the ratio of the light intensitiesin these states.

The switching time τ is determined using a photodiode by measuring thetime taken for the light signal to increase from a signal height of 10to 90%. The switching voltage comprises-rectangular pulses and is ±10V/μm.

The N* pitch is determined in wedge-shaped cells by the method ofGrandjean-Cano [F. Grandjean, CR Acad. Sci. (Paris) 172, 71 (1921); R.Cano, Bull. Soc. Franc. Mineral. Crystallogr. XC, 333 (1967)].

The phase-transition temperatures are determined using a polarizingmicroscope from the texture changes on heating. By contrast, the meltingpoint is determined using a DSC instrument. The phase-transitiontemperatures between the phases

    ______________________________________                                        Nematic              (N or N*)                                                Smectic-C            (S.sub.c or S.sub.c *)                                   Smectic-A            (S.sub.A or S.sub.A *)                                   Crystalline          (X)                                                      ______________________________________                                    

are given in °C., and the values are between the phase symbols in thephase sequence.

EXAMPLE 1

a) A ferroelectric liquid-crystalline mixture

    ______________________________________                                        5-octoxy-2-(4-butoxyphenyl)pyrimidine                                                                  24 mol %                                             5-octoxy-2-(4-hexoxyphenyl)pyrimidine                                                                  22.8 mol %                                           5-octoxy-2-(4-octoxyphenyl)pyrimidine                                                                  10.5 mol %                                           5-octoxy-2-(4-decoxyphenyl)pyrimidine                                                                  19.2 mol %                                           [4-(5-decylpyrimidin-2-yl)]phenyl                                                                      13.5 mol %                                           trans-4-pentylcyclohexanecarboxylate                                          4-(5-n-octylpyrimidin-2-yl)phenyl                                                                      10 mol %                                             (R)-tetrahydrofuran-2-carboxylate                                             (dope d)                                                                      ______________________________________                                    

exhibits the following liquid-crystalline phase ranges:

    X7S.sub.c *65.8S.sub.A *76.5N*92I

and has, at 25° C., a spontaneous polarization of 23 nC.cm⁻², a contrastof 7 and a switching time of 40 μs. The pitch of this mixture is +3.9 μmat a temperature of 90° C.

b) In comparison to this, the liquid-crystalline mixture claimed in DE3,831,226.3, which differs from the abovementioned ferroelectric mixtureonly in that it contains no dope, has the following phase ranges

    X9S.sub.c 84S.sub.A 93N105I

By comparison, the ferroelectric mixture has a melting point which islower by 2° C. EXAMPLE 2

a) A ferroelectric liquid-crystalline mixture comprising 6 components

    ______________________________________                                        5-octoxy-2-(4-butoxyphenyl)pyrimidine                                                                  24 mol %                                             5-octoxy-2-(4-hexoxyphenyl)pyrimidine                                                                  22.8 mol %                                           5-octoxy-2-(4-octoxyphenyl)pyrimidine                                                                  10.5 mol %                                           5-octoxy-2-(4-decoxyphenyl)pyrimidine                                                                  19.2 mol %                                           [4-(5-decylpyrimidin-2-yl)]phenyl                                                                      13.5 mol %                                           trans-4-pentylcyclohexanecarboxylate                                          (2S,3S)-2-[4-(5-octylpyrimidin-2-yl)-                                                                  10 mol %                                             phenyl]methyl-3-butyloxirane                                                  (dope e)                                                                      ______________________________________                                    

exhibits the following liquid-crystalline phase ranges:

    X6S.sub.c *81.4S.sub.A *88.7N*101I

and has, at 25° C., a spontaneous polarization of +17 nC.cm⁻², acontrast of 8 and a switching time of 30 μs. The pitch of this mixtureis -5.5 μm at a temperature of 90° C.

b) In comparison to this, the liquid-crystalline mixture claimed in DE3,831,226.3, which differs from the abovementioned ferroelectric mixtureonly in that it contains no dope, has the following phase ranges

    X9S.sub.c 84S.sub.A 93N105I

By comparison, the ferroelectric mixture has a melting point which islower by 3° C.

EXAMPLE 3

a) A ferroelectric liquid-crystalline mixture comprising 6 components

    ______________________________________                                        5-octoxy-2-(4-butoxyphenyl)pyrimidine                                                                  24 mol %                                             5-octoxy-2-(4-hexoxyphenyl)pyrimidine                                                                  22.8 mol %                                           5-octoxy-2-(4-octoxyphenyl)pyrimidine                                                                  10.5 mol %                                           5-octoxy-2-(4-decoxyphenyl)pyrimidine                                                                  19.2 mol %                                           [4-(5-decylpyrimidin-2-yl)lphenyl                                                                      13.5 mol %                                           trans-4-pentylcyclohexanecarboxylate                                          (2S,3R)-2-[4-(5-octylpyrimidin-2-yl)-                                                                  10 mol %                                             phenoxy]methyl-3-propyloxirane                                                (dope e)                                                                      ______________________________________                                    

exhibits the following liquid-crystalline phase ranges:

    X7S.sub.c *67S.sub.A *83.6N*96I

and has, at 25° C., a spontaneous polarization of +22 nC.cm⁻², acontrast of 8 and a switching time of 25 μs. The pitch of this mixtureis -2.6 μm at a temperature of 90° C.

b) In comparison to this, the liquid-crystalline mixture claimed in DE3,831,226.3, which differs from the abovementioned ferroelectric mixtureonly in that it contains no dope, has the following phase ranges

    X9S.sub.c 84S.sub.A 93N105I

By comparison, the ferroelectric mixture has a melting point which islower by 2° C.

EXAMPLE 4

a) A ferroelectric liquid-crystalline mixture comprising 6 components

    ______________________________________                                        5-octoxy-2-(4-butoxyphenyl)pyrimidine                                                                  24 mol %                                             5-octoxy-2-(4-hexoxyphenyl)pyrimidine                                                                  22.8 mol %                                           5-octoxy-2-(4-octoxyphenyl)pyrimidine                                                                  10.5 mol %                                           5-octoxy-2-(4-decoxyphenyl)pyrimidine                                                                  19.2 mol %                                           [4-(5-decylpyrimidin-2-yl)]phenyl                                                                      13.5 mol %                                           trans-4-pentylcyclohexanecarboxylate                                          (S)-4-(2-octoxypyrimidin-5-yl)phenyl                                                                   5 mol %                                              [spiro(1,3-dioxolane-2,1'-cyclohexane)-                                       4-yl]methyl ether                                                             (dope f)                                                                      ______________________________________                                    

exhibits the following liquid-crystalline phase ranges:

    X7S.sub.c *81S.sub.A *92N*102I

and has, at 25° C., a spontaneous polarization of -0.1 nC.cm⁻², acontrast of 7 and a switching time of 2.3 ms. The pitch of this mixtureis -25 μm at a temperature of 93° C.

b) In comparison to this, the liquid-crystalline mixture claimed in DE3,831,226.3, which differs from the abovementioned ferroelectric mixtureonly in that it contains no dope, has the following phase ranges

    X9S.sub.c 84S.sub.A 93N105I

By comparison, the ferroelectric mixture has a melting point which islower by 2° C.

EXAMPLE 5

a) A ferroelectric mixture comprising 7 components

    ______________________________________                                        Mixture from Example 1b) 90 mol %                                             (2S,3S)-2-[4-(5-octylpyrimidin-2-yl)-                                                                  7.7 mol %                                            phenoxy]methyl-3-butyloxirane                                                 4-(2-octoxypyrimidin-5-yl)phenyl                                                                       2.3 mol %                                            (2R,3R)-3-propyloxirane-2-carboxylate                                         (dope combination e.sub.1)                                                    ______________________________________                                    

has the following liquid-crystalline phase ranges:

    X4.5S.sub.c *80.5S.sub.A *88N*100.5I

At 25° C. this mixture has a spontaneous polarization of 18 nC.cm⁻², acontrast of 9 and, at a field strength of 10 V.μm⁻¹, a switching time of45 μs. This mixture has the particular advantage that the pitch iscompensated over the entire temperature range of the nematic phase.##STR26##

In addition, this ferroelectric mixture has the advantage that themelting point is 4.5° C. lower than in mixture 1b.

EXAMPLE 6

a) A ferroelectric liquid-crystalline mixture comprising the 9components

    ______________________________________                                        5-octoxy-2-(4-hexoxyphenyl)pyrimidine                                                                  18 mol %                                             5-octoxy-2-(4-octoxyphenyl)pyrimidine                                                                  7 mol %                                              5-octoxy-2-(4-butoxyphenyl)pyrimidine                                                                  17 mol %                                             5-octoxy-2-(4-decoxyphenyl)pyrimidine                                                                  9 mol %                                              5-octoxy-2-(4-dodecoxyphenyl)pyrimidine                                                                9 mol %                                              5-octyl-2-(4-dodecoxyphenyl)pyrimidine                                                                 14.5 mol %                                           [4-(5-decylpyrimidin-2-yl)]phenyl                                                                      21.5 mol %                                           trans-4-pentylcyclohexanecarboxylate                                          (S)-4-(2-octoxypyrimidin-5-yl)phenyl                                                                   1.6 mol %                                            [spiro(1,3-dioxolane-2,1'-cyclohexane)-                                       4-yl]methyl ether                                                             4-(2-octoxypyrimidin-5-yl)phenyl                                                                       2.4 mol %                                            (2R,3R)-3-propyloxirane-2-carboxylate                                         (dope combination f.sub.1)                                                    ______________________________________                                    

exhibits the following liquid-crystalline phase ranges:

    X-3S.sub.c *79S.sub.A *86N*100.5I

and has, at 25° C., a spontaneous polarization of +8.6 nC.cm⁻², acontrast of 8 and a switching time of 125 μs. The pitch of this mixtureis 60 μm at a temperature of 90° C.

b) In comparison to this, the liquid-crystalline mixture claimed in DE3,831,226.3, which differs from the abovementioned ferroelectric mixtureonly in that it contains no dope, has the following phase ranges

    X-2S.sub.c 81S.sub.A 86N104I

EXAMPLE 7

A ferroelectric mixture comprising the components

    ______________________________________                                        Mixture from Example 1b) 90 mol %                                             (R)-[2-(4'-dodecoxyphenyl)pyrimidin-5-yl]                                                              6.07 mol %                                           2,2-dimethyl-1,3-dioxolane-4-carboxylate                                      4-(2-n-octoxypyrimidin-5-yl)phenyl                                                                     3.93 mol %                                           (R)-tetrahydrofuran-2-carboxylate                                             (dope combination d.sub.1)                                                    ______________________________________                                    

has the following liquid-crystalline phase ranges

    X7.4S.sub.c *81S.sub.A *86N*100I

At 25° C., this mixture has a spontaneous polarization of 18 nC.cm⁻²and, in a switching field of 10 V.μm⁻¹, a switching time of 51 μs. Thecontrast of the measurement cell is 9. The melting point of this mixtureis 1.5° C. below that of the mixture from Example 1b. The pitch of thismixture is greater than 40 μm at 90° C.

EXAMPLE 8

A ferroelectric mixture comprising the components

    ______________________________________                                        Mixture from Example 1b) 90 mol %                                             4-(2-n-octoxypyrimidin-5-yl)phenyl                                                                     5.93 mol %                                           (R)-tetrahydrofuran-2-carboxylate                                             (2S,3R)-2-(4-(5-octylpyrimidin-2-yl)-                                                                  3.38 mol %                                           phenoxy]methyl-3-butyloxirane                                                 dope combination                                                              ______________________________________                                    

has the following liquid-crystalline phase ranges

    X5S.sub.c *72.5S.sub.A *87N*98I

At 25° C. this mixture has a spontaneous polarization of 23 nC.cm⁻² and,at a field strength of 10 V.μm⁻¹ a switching time of 42 μs. The meltingpoint of this mixture is 4° C. below that of the mixture from Example1b). The pitch of this mixture is greater than 21 μm at 90° C.

EXAMPLE 9

A ferroelectric mixture comprising the components

    ______________________________________                                        Liquid-crystalline mixture from Example 1b)                                                             94.3 mol %                                          4-(2-octoxypyrimidin-5-yl)phenyl                                                                        2.3 mol %                                           (2R,3R)-4-propyloxirane-2-carboxylate                                         (2S,3R)-2-[4-(5-octylpyrimidin-2-yl)phenoxy]-                                                           3.4 mol %                                           methyl-3-butyloxirane                                                         (dope combination g)                                                          ______________________________________                                    

exhibits the following liquid-crystalline phase ranges:

    X7S.sub.c *70S.sub.A *85N*99I

At 25° C., this mixture has a spontaneous polarization of 19 nC cm⁻²and, at a field strength of 10 V.μm⁻¹, a switching time of 32 μs. Themelting point of this mixture is 4° C. below the melting point of themixture from Example 1b). The pitch of this mixture is greater than 50μm at 90° C.

Compared with the mixture from Example 5, this mixture has a switchingtime which is shorter by 13 μs and has a higher spontaneous polarizationat a lower overall dope concentration.

These results confirm the advantages of the use of oxirane ethers havinga (2S,3R) configuration compared with oxirane ethers having a (2R,3R)configuration.

EXAMPLE 10

a) A ferroelectric mixture comprising the components

    ______________________________________                                        5-octyl-2-(4-heptonoyloxyphenyl)-                                                                      17.6 mol %                                           pyrimidine                                                                    5-octoxy-2-(4-exoxyphenyl)pyrimidine                                                                   11.8 mol %                                           5-octyloxy-2-(4-octoxyphenyl)pyrimidine                                                                4 mol %                                              5-octoxy-2-(4-butoxyphenyl)pyrimidine                                                                  13 mol %                                             5-octoxy-2-(4-decoxyphenyl)pyrimidine                                                                  7 mol %                                              5-octoxy-2-(4-dodecoxyphenyl)pyrimidine                                                                7 mol %                                              5-octyl-2-(4-dodecoxyphenyl)pyrimidine                                                                 10.5 mol %                                           [4-(5-decylpyrimidin-2-yl)]phenyl                                                                      16.7 mol %                                           trans-4-pentylcyclohexanecarboxylate                                          (S)-4-(2-octoxypyrimidin-5-yl)phenyl                                                                   5.4 mol %                                            [spiro(1,3-dioxolane-2,1'-cyclohexane)-                                       4-yl]methyl ether                                                             4-(2-octoxypyrimidin-5-yl)phenyl                                                                       6.6 mol %                                            (2R,3R)-3-propyloxirane-2-carboxylate                                         (dope combination f.sub.1)                                                    ______________________________________                                    

exhibits the following liquid-crystalline phase ranges:

    X-8S.sub.c *65S.sub.A *74N*86I

and has, at a temperature of 25° C., a spontaneous polarization of 30nC.cm⁻² and a switching time of 41 μs.

This mixture has the advantage that the pitch is compensated over theentire temperature range of the nematic phase. ##STR27##

b) In comparison to this, a liquid-crystalline mixture which differsfrom the abovementioned ferroelectric mixture only in that it containsno dope has the following phase ranges

    X-5S.sub.c 71S.sub.A 78N94I

The dopes also have the advantage that they lower the melting point.

EXAMPLE 11

a) A ferroelectric mixture comprising the following components

    ______________________________________                                        Mixture from Example 6b) 76.5 mol %                                           (2S,3S)-2-(4-(5-octylpyrimidin-2-yl)-                                                                  18 mol %                                             phenoxy]methyl-3-butyloxirane                                                 4-(2-octoxypyrimidin-5-yl)phenyl                                                                       5.5 mol %                                            (2R,3R)-3-propyloxirane-2-carboxylate                                         (dope combination e.sub.1)                                                    ______________________________________                                    

has the following liquid-crystalline phase ranges:

    X-1S.sub.c *65S.sub.A *68N*83I

At a temperature of 25° C., this mixture has a spontaneous polarizationof 50 nC.cm ⁻² and, at a field strength of 10 V.μm⁻¹, a switching timeof 50 μs. This mixture has the particular advantage that the pitch iscompensated over the entire temperature range of the nematic phase.##STR28##

EXAMPLE 12

a) A ferroelectric mixture comprising 9 components

    ______________________________________                                        Mixture from Example 6b) 96.2 mol %                                           (2S,3S)-2-[4-(5-octylpyrimidin-2-yl)-                                                                  2.9 mol %                                            phenoxy]methyl-3-butyloxirane                                                 4-(2-octoxypyrimidin-5-yl)phenyl                                                                       0.9 mol %                                            (2R,3R)-3-propyloxirane-2-carboxylate                                         (dope combination e.sub.1)                                                    ______________________________________                                    

has the following liquid-crystalline phase ranges:

    X1S.sub.c *66S.sub.A *74N*90I

At 25° C. this mixture has a spontaneous polarization of 5.8 nC cm⁻²and, at a field strength of 10 V μm⁻¹, a switching time of 114 μs. Thepitch in the nematic phase is >30 μm at a temperature of 80° C.

EXAMPLE 13

a) A ferroelectric mixture comprising the components

    ______________________________________                                        Mixture from Example 10b)                                                                             95.65 mol %                                           (2S,3S)-2-[4-(5-octylpyrimidin-2-yl)-                                                                 2.8 mol %                                             phenoxy]methyl-3-propyloxirane                                                4-(2-octoxypyrimidin-5-yl)phenyl                                                                      1.55 mol %                                            (2R,3R)-3-propyloxirane-2-carboxylate                                         (dope combination e.sub.1)                                                    ______________________________________                                    

has the following liquid-crystalline phase ranges:

    X-4S.sub.c *67S.sub.A *75N*91I

At 25° C. this mixture has a spontaneous polarization of 10.3 nC.cm⁻²,and, at a field strength of 10 V.μm⁻¹, a switching time of 76 μs. Thepitch of this mixture is greater than 10 μm at a temperature of 80° C.

EXAMPLE 14

a) A ferroelectric mixture comprising the 9 components

    ______________________________________                                        5-octoxy-2-(4-hexoxyphenyl)pyrimidine                                                                  10.3 mol %                                           5-octoxy-2-(4-octoxyphenyl)pyrimidine                                                                  3.5 mol %                                            5-octoxy-2-(4-butoxyphenyl)pyrimidine                                                                  11.5 mol %                                           5-octoxy-2-(4-decoxyphenyl)pyrimidine                                                                  6.3 mol %                                            5-octyl-2-(4-dodecoxyphenyl)pyrimidine                                                                 9.2 mol %                                            [4-(5-decylpyrimidin-2-yl)]phenyl                                                                      14.8 mol %                                           trans-4-pentylcyclohexanecarboxylate                                          [4-(5-dodecylpyrimidin-2-yl)]phenyl                                                                    7.9 mol %                                            trans-4-pentylcyclohexanecarboxylate                                          5-octyl-2-(4-heptonoyloxyphenyl)pyri-                                                                  15.5 mol %                                           midine                                                                        4-(2-(4-butyldimethylsilylbutoxy-                                                                      3 mol %                                              pyrimidin-5-yl)phenyl (2R,3R)-3-                                              hexyloxirane-2-carboxylate                                                    (2S,3S)-3-butyloxirane-2-methyl                                                                        18 mol %                                             2-(4-(4-butyldimethylsilylbutoxy)-                                            phenyl)pyrimidin-5-yl ether                                                   (dope combination e.sub.1)                                                    ______________________________________                                    

exhibits the following liquid-crystalline phase ranges:

    X-11S.sub.c *71S.sub.A *75N*86I

and has, at a temperature of 25° C., a spontaneous polarization of 47nC.cm⁻² and a switching time of 80 μs. The pitch of this mixture is 8 μmat a temperature of 78° C.

b) In comparison to this, a liquid-crystalline mixture which differsfrom the abovementioned ferroelectric mixture only in that it containsno dope has the following phase ranges

    X-2S.sub.c 75.2N100I

The dopes thus have the advantage that they additionally lower themelting point.

We claim:
 1. A ferroelectric liquid-crystalline mixture containing, ascomponent A, at least two 5-alkoxy-2-(alkoxyphenyl)pyrimidines of thegeneral formula (I) ##STR29## in which the alkyl radicals arestraight-chain alkyl radicals in which n is an integer from 6 to 14 andx is an integer from 2 to 14, and, optionally, one or morecyclohexanecarboxylates of the general formula (II) ##STR30## in whichR¹ is an alkyl radical having 10 to 16 carbon atoms or an alkoxy radicalhaving 8 to 14 carbon atoms, and R² is an alkyl radical having 2 to 9carbon atoms, and, optionally, one or more alkenyloxyphenylpyrimidinederivatives of the general formula (III) ##STR31## in which R³ is analkyl chain having 7 to 16 carbon atoms or an alkoxy chain having 6 to14 carbon atoms, and y is an integer from 4 to 14, and, optionally, oneor more alkylpyrimidinealkoxyphenyl derivatives of the general formula(IV) ##STR32## in which m and p, independently of one another, areintegers from 6 to 14, and as component B, an optically active compoundfrom the groupa) optically active esters of 1,3-dioxolane-4-carboxylicacids, and b) optically active esters of oxirane-2-carboxylic acids,andoptionally containing, as component C, one or more carboxylates of thegeneral formula: (V), (VI), (VII), (VIII) or (IX): ##STR33## where k maybe an integer from 6 to 14, and l may be an integer from 2 to 14,##STR34## where m may be an integer from 5 to 14, and l may be aninteger from 2 to 14, ##STR35## where p may be an integer from 7 to 14,and r may be an integer from 4 to 14, ##STR36## where s may be aninteger from 6 to 14, and t may be an integer from 6 to 14, ##STR37##where R² is straight-chain or branched (C₁ -C₁₂)alkyl or alkenyl, itbeing possible for one or two non-adjacent CH₂ groups to be replaced byO and/or S atoms, y is F, Cl, Br, CN or CF₃ and R¹ is branched (C₃ -C₉)alkyl, benzyl or phenyl, and an optically active component D whichcomprises one or more compounds from the following groups (X) and/or(XI) and/or (XII),Group (X): optically active esters oftetrahydrofuran-2-carboxylic acid with mesogenic phenols Group (XI):optically active aryl 2,3-epoxyalkyl ethers of mesogenic phenols Group(XII): optically active 1,3-dioxolan-4-yl ethers of mesogenic phenols,and in which the pitch of the helix (induced in the nematic phase) isgreater than 10 μm or infinite.
 2. A ferroelectric liquid-crystalmixture as claimed in claim 1, which contains 0.5 to 30 mol-% of theoptically active component D.
 3. A ferroelectric liquid-crystal mixtureas claimed in claim 1, which contains two dopes from Groups D and B. 4.A ferroelectric liquid-crystal mixture as claimed in claim 1, whichcontains, as component D, a chiral tetrahydrofuran-2-carboxylate (X) ofthe following general structure (Xa): ##STR38## where R² is astraight-chain or branched alkyl or alkenyl radical having 6 to 12carbon atoms, which may contain an asymmetric carbon atom;-M³ is --O,--S, --O--CO or --CO, K is zero or 1, -A⁴ is ##STR39##
 5. Aferroelectric liquid-crystal mixture as claimed in claim 1, whichcontains, as component D, a chiral epoxyalkyl ether of the generalstructure (XIa) ##STR40## (aryl 2,3-epoxyalkyl ether) R¹ isstraight-chain or branched (C₁ -C₁₂) alkyl, it being possible for one ortwo non-adjacent CH₂ groups to be replaced by O and/or S atoms,A isdiazine-2,5-diyl or diazine-3,6-diyl, and R², R³ and R⁴ independently ofone another, are H, straight-chain (C₁ -C₁₀)alkyl or branched (C₃-C₁₀)alkyl, where R², R³ and R⁴ are not simultaneously H.
 6. Aferroelectric liquid-crystal mixture as claimed in claim 1, whichcontains, as component D, a chiral dioxolan-4-yl ether of the structure(XIIa) ##STR41## in which: R⁶ and R⁷ are methyl or, together with theC(2) atom of the dioxolane ring, are a cyclohexane ring,R⁵ is astraight-chain or branched alkyl or alkylene radical having 6 to 12carbon atoms, which may contain an asymmetric carbon atom, -M³ is --O,--S or --O--CO, K is zero or 1, and -A⁴ is ##STR42##
 7. A ferroelectricliquid-crystal mixture as claimed in claim 1, which contains, ascomponent D, one or more compounds of the structure (Xb), (XIb) and(XIIb): ##STR43## ##STR44## where A is pyrimidine-2,5-diyl, andR¹ is C₁-C₁₀ -alkyl, it being possible for one or two non-adjacent CH₂ groups tobe replaced by O.
 8. An electrooptical assembly containing aferroelectric mixture as claimed in any one of claims 1 to
 7. 9. Aferroelectric liquid-crystal mixture as claimed in claim 1, whichcontains, as component D, 4-(5-n-octylpyrimidin-2-yl)phenyl(R)-tetrahydrofuran-2-carboxylate.